The invention relates to a patient monitor, and particularly, to a patient monitor having a compliance monitoring circuit for indicating a degradation in the connection of the patient monitor to a patient.
Compliance monitoring circuits are used in a variety of environments. In general, the term xe2x80x9ccompliancexe2x80x9d is used to indicate the ability of a current source to match (i.e., supply current to) a changing load. The term xe2x80x9ccompliancexe2x80x9d in the medical field is used to indicate the ability of the current source of the patient monitor to match the changing impedance of the patient connection. The electrical impedance between the medical device and the patient varies greatly depending upon patient temperature, skin moisture content, electrode adhesion, etc. In a patient monitoring device, the compliance can be monitored to provide an indication of the quality of the connection between the medical device and the patient.
One example of a compliance monitoring circuit is disclosed in U.S. Pat. No. 5,800,458 to Wingrove. Wingrove discloses using the compliance monitor as a timer in a stimulator unit for electrotherapy. As electrotherapy is being applied to a subject, the timer of the compliance monitor keeps track of the amount of time therapy is being applied. If the compliance monitor has a degrading connection to the subject (such as the subject removing the instrument), the compliance circuit will stop the timer.
Another example of a compliance monitoring circuit is in a patient monitor used for determining whether neuromuscular blocking agents in anesthesia are still active (commonly referred to as a neuromuscular monitor). Neuromuscular blocking agents are used to paralyze specific muscles of the body to allow a clinician to perform specific medical procedure such as intubation (the process of placing a tube into the patient""s trachea to establish an airway), or to generally anesthetize patients during general surgical procedures. To eliminate movement of the patient, a large dose of neuromuscular blocking drug is given to completely paralyze the patient.
Depending on the amount of drug given, different degrees of neuromuscular block can be achieved. Some surgical processes require very little paralyzation while others require long periods of intense block. A commonly used method for monitoring the degree of block involves applying a small electrical current to the patient""s skin (near a nerve) and noting the response of the associated muscle. Typically the current is delivered to the wrist (ulnar nerve) and the thumb moves in response. With a deep block, the thumb may not move at all; with no block, the thumb""s movement is quite pronounced; and with a shallow block, the thumb""s movement is decreased. A trained anesthesiologist can gauge the thumb movement by feel and adjust the administration of blocking drugs accordingly. The device which applies the stimulus is usually battery powered and delivers currents in range of 50 mA.
Problems arise, however, if the patient impedance is high or if the connection of the patient monitor to the patient is poor (e.g. if the electrode is falling off or has fallen off of the patient). If the patient impedance is high, or if the patient connection is poor, the electrical stimulus may be insufficient to generate a muscular response. Stated differently, the current source of the patient monitor may not have enough power to match the changing impedance of the patient connection. Under these circumstances, a clinician may wrongly assume an adequate state of anesthetization exists. Therefore, a compliance monitoring circuit is required to monitor the quality of the connection of the patient monitor to the patient.
Accordingly, the invention provides a patient monitor and a means for evaluating the connection between the patient monitor and the patient. The means for evaluating the connection includes a compliance monitoring circuit. The compliance monitoring circuit includes a constant current generator delivering a constant current to an isolation transformer, and a signal generator connected to the primary winding to generate a signal in response to a change in the connection to the patient. The input voltage to the constant current generator can be any type of voltage waveform including a square wave. The isolation transformer has a primary winding connected to the current generator and a secondary winding connected to a patient by electrodes. The power being supplied to the primary winding is reflected to the secondary winding at a different voltage and current. Furthermore, the impedance of the load will be reflected from the secondary winding to the primary winding.
The signal generator of the compliance monitoring circuit further includes alarm signal circuitry connected to the constant current generator. The alarm signal circuitry provides an alarm signal to indicate when there is a degradation in the connection of the electrode to the patient or where one of the electrodes is accidentally removed or otherwise becomes disconnected. This will result in a high or even infinite load resistance
The invention further provides a method for monitoring the compliance of a connection between a patient monitoring device and a patient. The monitoring of the compliance includes the acts of providing an isolation transformer connected between the patient monitor and the electrode, generating a constant current in the primary winding using a constant current generator connected to the primary winding, monitoring the compliance voltage of the constant current generator to determine compliance of the connection between the electrode and the patient, and generating a signal in response to a change in the connection to the patient.
In one embodiment, the monitoring of the compliance voltage includes the act of generating a signal, which may be used as an alarm signal, to indicate a degradation in the connection between the electrode and the patient. In still another embodiment, the monitoring of the compliance voltage also includes the act of comparing the compliance voltage to a reference voltage. If the compliance voltage is greater than the reference voltage, the alarm signal will be generated.
It is a principal advantage of the invention to provide a patient monitor and a compliance alarm circuit therefor for measuring the load on the secondary without the use of complex circuitry.
It is another advantage of the invention to provide a patient monitor and a compliance monitor therefor that generates a signal, which may be used as an alarm signal, when there is degradation in the connection of the compliance monitoring circuit or patient monitor to the patient.
It is another advantage of the invention to provide a patient monitor and a compliance alarm circuit therefor that takes advantage of the isolation transformer""s ability to reflect the impedance of the load from the secondary winding to the primary winding.
It is another advantage of the invention to provide a method of measuring the compliance of a connection between an electrode of a patient monitoring device and a patient by monitoring a compliance voltage of a constant current generator.
It is another advantage of the invention to provide a method of measuring the compliance of a connection between an electrode of a patient monitoring device and a patient by generating a signal, which may be used as an alarm signal to indicate a when a degradation in the connection between the electrode and the patient.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.